Manufacturing process for a stacked structure comprising a thin layer bonding to a target substrate

ABSTRACT

A process for manufacturing a stacked structure comprising at least one thin layer bonded to a target substrate, in which a thin layer is formed by introduction gaseous species into an initial substrate, to form a weakened layer separating a film from the rest of the initial substrate, a first contact face of the thin layer is bonded to a face of an intermediate substrate by molecular adhesion, and the initial substrate is fractured at the weakened layer so as to expose a free face of the thin layer. The intermediate substrate is then removed in order to obtain the stacked structure.

TECHNICAL FIELD

This invention relates to a process for manufacturing a stackedstructure comprising a thin layer bonding to a target substrate. Theinvention is particularly applicable to the field of semiconductors.

STATE OF PRIOR ART

Document FR-A-2 681 472 (corresponding to American U.S. Pat. No.5,374,564) discloses a process for manufacturing thin films made of asemiconducting material. This document divulges that implantation of arare gas and/or hydrogen in a substrate made of a semiconductingmaterial could create a weakened layer that could contain micro-cavitiesor micro-bubbles (or platelets) at a depth approximately equal to theaverage penetration depth of the implanted ions. The implanted face ofthis substrate is brought into intimate contact with a support acting asa stiffener. Furthermore, heat treatment can be applied at asufficiently high temperature to induce an interaction (or coalescence)between the micro-cavities or micro-bubbles causing a separation orfracture of the semiconducting substrate into two parts, namely a thinsemiconducting film bonding to the stiffener, and secondly the rest ofthe semiconducting substrate that may in its turn be recycled as adonating substrate or a support. The separation takes place at thelocation at which the micro-cavities or micro-bubbles are present, inother words along the micro-cavities layer. The heat treatment is suchthat the interaction between the micro-bubbles or the micro-cavitiescreated by implantation causes a separation between the thin film andthe rest of the substrate. Therefore, there is a transfer of a thin filmfrom an initial substrate as far as a stiffener that acts as a supportfor this thin film.

Ionic implantation refers to any means of introducing previously definedcompounds alone or in combination. Some examples are ionic bombardment,diffusion, etc.

This process may also be applied to the manufacture of a thin film madeof solid material other than a semiconducting material (a conducting ordielectric material) that may or may not be crystalline. This film maybe single layer of multi-layer (for example see document FR-A-2 748850).

This process can be used to make platelets, for example electronicquality SOI wafers, at an attractive cost, by transferring a siliconfilm onto a silicon substrate covered by an oxide layer.

The article “A new characterization process used to qualify SOI films”by H. MORICEAU et al published in E.C.S. Proc. Vol. 99-3, p. 173,discloses that an SOI wafer made according to this type of process canbe bonded onto an oxidized silicon substrate. The result after thesilicon substrate has been removed from the SOI wafer and from its oxidelayer by means of a selective chemical attack (for example based on HF)is another SOI wafer with an inverted silicon film since the free faceof this film is the face that was bonded to the buried oxide layer ofthe initial SOI wafer. This procedure was used because it was possibleto examine defects in the silicon film. The purpose of this study was toidentify the position of defects within the film thickness, rather thanto make a new structure.

The articles entitled “Evaluation of defects in surface Si near Si/BOXinterface in SIMOX platelets” by M. SUDOU et al. published in E.C.S.Proc. Vol. 97-23, p. 119 and “SIMOX technology and applications to waferbonding” by A. J. AUBERTON-HERVE et al. published in E.C.S. Proc. Vol.95-7, p. 12 disclose experiments with bonding an SOI wafer obtainedusing the SIMOX process directly on a silicon substrate or pure silicasubstrate. This process comprises a single bonding step and is followedby removal of the substrate from the SOI wafer. The purpose of theseexperiments was also to examine defects in the silicon film. A similarapproach was used in the article entitled “Ultra thin silicon filmsdirectly bonded onto silicon wafers” by F. FOURNEL et al., published inMaterials Science and Engineering, B 73 (2000), pages 42 to 46 to make adislocation network in an interface plane defined when a thin film and amonocrystalline silicon substrate are put into contact. The article doesnot provide any information about the means of obtaining the thin layerin the stacked structure as described by the invention.

Document FR-A-2 725 074 (corresponding to American U.S. Pat. No.5,863,830) discloses a process for manufacturing a structure comprisinga thin semiconducting film bonding to a target substrate. Initially, thethin film is bonded to an initial substrate by a first bonding energy.The thin film is then transferred from the initial substrate to thetarget substrate by applying tear off forces to overcome the firstbonding energy and bonding of the thin film on the target substrate.This thin film may be transferred by means of an intermediate ormanipulator substrate that in turn is separated from the thin film bytearing off. This process requires that the bonding energies at thedifferent bonding interfaces associated with the thin film must be wellcontrolled to enable successive tearing off.

The technique known as BESOI does not use separation of a thin film byimplantation of gaseous compounds. One of its main characteristics is touse a crystalline stop layer to enable chemical (selective) attack whilesupporting a monocrystalline layer, typically made of silicon. This stoplayer is a monocrystalline layer or an epitaxied layer. It may be adoped silicon layer, a layer obtained by epitaxy of crystalline materialwith a nature different from the nature of the thin film (for exampleSi_(x)Ge_(1-x)), a monocrystalline layer of silicon made porous formedfrom the solid part of the silicon substrate. This technique cannot makeuse of an amorphous stop layer if it is required to deposit amonocrystalline film on this stop layer. It does not provide anyinformation about the use of an intermediate substrate.

With the known process for making thin layers of structures, called thedirect process and described in FR-A-2 681 472 and based on hydrogenimplantation, molecular bonding with a final support and separation inand/or close to the implanted area, it is difficult to obtain somestructures with specific properties, and/or these structures cannot beobtained with a sufficient quality. In the following cases, the directprocess cannot be easily applied and the technical solution to beprovided as described in this invention is not simply an adaptation ofthe process.

A first case relates to the manufacture of structures of thin and/orvery thin stacked films with a crystalline layer present on the surfaceof an amorphous layer. A first example is the manufacture of films withthicknesses less than a few tens of nanometers, for example 20 nm thicksilicon films on 20 nm thick SIO₂ films bonding to a solid siliconsupport. Bonding defects are then frequently revealed during theseparation step in the direct process, which can be demonstrated whenthe process uses a separation step involving a low temperature heattreatment (for example less than 500° C.); this separation step may ormay not be mechanically assisted. A second example is the case in whichthin film structures with specific properties are subjected to heattreatments at temperatures greater than the temperatures of heattreatments applied previously during or after thinning of one of thesubstrates used (for example using thinning by the direct process).Bonding defects, for example such as bubbles inflated with gas, can thenoccur to the detriment of the quality of the structure.

A second case relates to the manufacture of structures in which bondingforces before the separation step using the direct process are very low.For example, for some surface preparation conditions before putting intocontact (cleaning, level of the component manufacturing process thatgenerates a given surface roughness), the bond between the final supportand the generating plate of the film to be transferred has a very lowenergy. The separation step in the direct process is then impossible.

A third case relates to the manufacture of stacked structures ofmaterials for which the coefficients of thermal expansion are toodifferent. If the difference between the coefficients of thermalexpansion of the material from which the film to be transferred is made,and the material of the final support is too high, the bond will failbefore separation by the direct process if a heat treatment is appliedto the bond. For example, this is the case for silicon and sapphire, forwhich the ratio between the coefficients of thermal expansion is 2.

A fourth case relates to the manufacture of structures in which bondingforces after the separation step using the direct process must be low orvery low. It may be desirable for the bonding energy of the stackedstructure to remain low after the separation step using the directprocess, possibly even lower than the force necessary for separation,such that separation will be possible at the bonding interface later.This is applicable particularly when several treatments can reinforcethe bonding interface. In a first example that for example correspondsto a removable target substrate so that it can be reused, thinning bysacrificial oxidation of a silicon surface film of an SOI structure at950° C. causes an increase in the bonding energy greater than 1 J/m²which is not conducive to subsequent separation of the stacked structureof the target substrate. In a second example, thermal diffusion ofcompounds, local oxidation, etc., may be necessary to treat all or someof the layers, which is not a positive point for separation of thestacked structure since these operations participate in reinforcing thebonding energy. It may also be desirable to have a very low bondingenergy in the final structure, for example in the case in which it isrequired to make a deposit on the structure, this deposit being highlystressed compared with all or some of the stacked structure and thetarget substrate. This weak interface then acts as a stressaccommodation area. This is a support compliance application.

A fifth case relates to the manufacture of stacked structures made ofheterogeneous materials. The use of materials with different natures,for example silicon or thermal oxide (various dielectrics, metallicmaterials, semiconductors, superconductors, etc.) may cause bondingdefects demonstrated in the direct process. For example, the bonding ofa silicon film covered by a silicon nitride film, with a silicon nitridefilm itself covering a silicon film, frequently introduces bondingdefects that are easily demonstrated during the separation step in thedirect process. In this case, a film is a substrate or a film or astacked structure covered on the surface by the mentioned film.

A sixth case applies to the manufacture of stacked structures in which aphase change or a change in the nature of a material may occur. Forexample, some materials cannot be used in direct process due toincompatibility with the thermal budgets. Thus a stacked structurecomposed of a silicon film, a palladium film and a silicon waferproduces a suicide above 200° C. that can give a good bond. The bonddegrades at higher temperatures, for example above 900° C. which is thetemperature typically used to oxide a silicon film in order to reduceits thickness. Another example applies to optical applications in whicha metallic mirror may be added to a silicon sheet by molecular bonding.This metallic mirror cannot be heat treated at temperatures above a fewtens of ° C., so that thermal budgets that could be used in theseparation process would be impossible.

The direct process cannot always be used to make stacked film structuresincluding the conservation of a specific surface, called the front face.

In this respect, note as a first example the manufacture of stackedstructures in which it is difficult to polish the surface film obtained.For example, the surface roughness of the structure thinned by thedirect process after the separation step will need to be reduceddepending on the planned application. In the case of silicon, thisreduction in the roughness may conventionally be obtained bychemical-mechanical polishing (CMP). For many materials, for example“hard” materials, this polishing is either not suitable (ineffective) ortakes too long (higher industrial cost). This is the case of a structurethinned using the direct process terminated at the surface using asapphire, SiC or diamond film, for example. The surface micro-roughnessof the “hard” film has to be reduced to satisfy the requiredapplication. For this type of material, polishing by CMP takes a verylong time to implement and the uniformity of polishing on the structureis very difficult to control. The extra cost involved is then very highif an epi-ready type quality is to be achieved.

We can also mention the manufacture of stacked structures in which oneof the films is provided with one different characteristic on the twofaces. This is the case if the structure obtained by the direct processcomprises a surface incompatible with the planned used after theseparation step. For example, due to its polar nature, a monocrystallineSiC film has the characteristic that the surface of one face is composedmainly of silicon atoms on one face (called an Si type surface) and thesurface of the other face is composed mainly of carbon atoms (C typesurface). Continued growth in epitaxy on SiC assumes that a free Si typesurface is available. However the transfer of an SiC film, for exampleusing the direct process, is accompanied by a change in the nature ofthe surface due to turning over. The initial free surface is of the Sitype since this type of face is easy to polish with SiC and is easy tobond by molecular bonding. Therefore, the free face is of the C typeafter transfer by the direct process. The same is true for a GaN layer.

DESCRIPTION OF THE INVENTION

This invention overcomes the disadvantages of prior art and provides ameans of creating a stacked structure comprising either a film with somespecific properties, or a film with a least one surface with specificproperties.

The invention is based on the surprising fact observed by the inventors,that unlike the direct process for which the thin layer is bonded to thetarget substrate and then thinned, all the defects mentioned above canbe avoided if the initial substrate is thinned before the thin layer isbonded to the target substrate. One or several intermediate supportsneed to be used to invert these bonding and thinning steps.

Therefore, the purpose of the invention is a process for manufacturing astacked structure comprising at least one thin layer bonding to a targetsubstrate, comprising the following steps:

a) formation of a thin layer starting from an initial substrate, thethin layer having a free face called the first contact face,

b) putting the first contact face into bonding contact with a face of anintermediate support, the structure obtained being compatible with laterthinning of the initial substrate,

c) thinning of the said initial substrate to expose a free face of thethin layer called the second contact face and opposite the first contactface,

d) putting a face of the target substrate into bonding contact with atleast part of the second contact face, the structure obtained beingcompatible with later removal of all or some of the intermediatesupport,

e) removal of at least part of the intermediate support in order toobtain the said stacked structure.

Thinning in step c) or removal in step e) can use any technique capableof eliminating the initial substrate or the intermediate support. Inparticular, separation, fracture (by the creation of an area weakened bythe introduction of gaseous compounds), mechanical and/or chemicalattack. It may be possible to reuse the substrate and the support,depending on the thinning and removal type.

According to one particular embodiment, the target substrate is only atemporary support for the thin layer, the said steps in the processbeing entirely or partly repeated, the target substrate being treated asthe initial substrate or the intermediate support.

Thus, the process according to the invention can be used to transfer thethin layer from one support to another support as many times asnecessary to obtain a stacked structure with the requiredcharacteristics, and particularly a stack with technological componentlevels.

The compatibility of the said structure in step b) and/or in step d) maybe achieved by forming a thin layer in step a), in order to avoidbonding defects during thinning in step c) and removal in step e),respectively. This compatibility may be due to the finished thickness ofthe thin layer and/or the material or materials making up the said thinlayer. The nature of the intermediate support and/or the targetsubstrate in contact with the thin layer, may be chosen to avoid anyincompatibility related to a phase change of the materials in theresulting structure. The nature of the intermediate support and/or thetarget substrate in contact with the thin layer, may be chosen so as toavoid any incompatibility related to heterogeneity of the materials inthe resulting structure. The nature of the intermediate support and/orthe target substrate in contact with the thin layer, may be chosen toavoid any incompatibility related to a difference between thecoefficients of thermal expansion of the intermediate support and thethin layer. To enable this compatibility, the thin layer and/or theintermediate support and/or the target substrate may comprise at leastone additional layer with contact face(s). In this case, before step d),the additional layer may be provided with all or part of at least onecomponent. The additional layer may be composed of an oxide orpolycrystalline silicon or amorphous silicon.

Steps a) and c) may be such that the roughnesses of the first contactface of the thin layer and/or the intermediate support are less than theroughnesses of the second contact face and/or the target substraterespectively, the compatibility of the structure of step d) beingobtained by putting the second contact face of the thin layer intobonding contact and by removal of the intermediate support.

The bonding contact of the first contact face and/or the second contactface of the thin layer achieving the said compatibility in step b)and/or step d) may result from the use of a treatment enabling bondingcontact. The treatment enabling bonding contact may bemechanical-chemical and/or ionic polishing, insertion of an intermediatelayer between a corresponding contact face of the thin layer and theintermediate support or the target substrate, or a heat treatment or achemical treatment, or any combination of these treatments may beapplied. The treatment may be applied at high temperature due tocompatibility of the structure.

Advantageously, step b) and/or step d) may be put into bonding contactby molecular bonding.

The surface polarity (related to the nature of the atoms on the surface)of the first contact face of the thin layer may be different from thesurface polarity of the second contact face, the compatibility of thestructure of step d) being obtained by putting the second contact faceof the thin layer into bonding contact with the target substrate, and byremoval of the intermediate support from the first contact face of thethin layer that thus becomes a free face. The structure compatibilityachieved in step d) may be obtained by putting the second contact faceof the thin layer into bonding contact with the target substrate with abonding energy appropriate for possible removal of the target substrateafter step e). Advantageously, this bonding energy is low. Anintermediate step may be inserted between steps c) and d), consisting ofmaking elements in the second contact face of the thin layer and/or inthe target substrate, the structure obtained after step d) beingcompatible with the presence of the said elements. These elements may becavities or all or part of components (microelectronics, optronics,etc.) and may be made with a certain surface topology. Before step d),an intermediate step may be provided consisting of a trimming operationin order to isolate at least one area of the second contact face, stepd) putting at least one of these areas into bonding contact with thetarget substrate. The trimming operation may possibly take place beforestep a). After the removal step e), the area(s) of the layer that arenot in bonding contact with the target substrate remain on theintermediate support and may be transferred later.

Step a) may be done starting from a substrate covered by at least onelayer of material. In this case, after step e), the process may comprisea step consisting of eliminating the layer of material covering thesubstrate in step a).

According to one preferred embodiment:

-   -   step a) includes the introduction of gaseous compounds into the        initial substrate through one of its faces corresponding to the        first said contact face, to form a weakened layer separating the        said film from the rest of the initial substrate and possibly        leading to a fracture of the initial substrate during step c),    -   step c) consists of making a preferred treatment to obtain the        fracture in the initial substrate at the weakened layer.

This treatment may be thermal and/or mechanical. The initial substratemay be single layer or multi layer. In particular, it may comprise anepitaxied layer. The same applies for the target substrate and for theintermediate support.

According to one preferred embodiment, at least part of the intermediatesupport may be removed by the introduction of gaseous compounds eitherthrough the thin layer after creating a contact, or through the contactface of the intermediate support before or after putting it into bondingcontact with the first contact face of the thin layer, this addition ofgaseous compounds forming a weakened layer enabling the removal of allor part of the intermediate support, possibly covered by a film of thethin layer, by fracture. In this case, the intermediate support may bereused, for example, as a new support.

Possibly, the stacked structure obtained at the end of step e) isthinned on the side of the first contact face.

The process may use a very good quality and therefore high cost initialsubstrate, for example a 300 mm diameter monocrystalline silicon, anintermediate support compatible with the initial substrate in the senseof step b), for example a monocrystalline silicon substrate covered byan SiO₂ oxide film, a target substrate made of polycrystalline ofmonocrystalline silicon of lower quality than the silicon in the initialsubstrate, for example the thin layer comprising silicon oxide on themonocrystalline silicon originating from the initial substrate.Similarly, the target substrate may be other than silicon. At the end ofthe process, the film obtained on the target substrate is then very goodquality. Furthermore, the initial substrate may be made, and theintermediate support will be reusable or may be sacrificial depending onits quality or cost. Furthermore, an SiC or GaAs initial substrate, anSiC or GaAs intermediate support, or an SiC or GaAs target substrate oflower quality than the initial substrate material may be used, the thinlayer containing SiC or GaAs originating from the initial substrate.

The thin layer may also be a layer of a material from Si, GaN, SiC,LiNb0 ₃, Ge, GaAS, InP, sapphire and semiconductors.

The invention provides many advantages including the following:

A monocrystalline film with good crystalline quality can be transferred,while the stop layer for thinning the intermediate support is anamorphous layer.

A recyclable intermediate support may be used, for example by checkingits bonding energy, if the cost of the intermediate substrate (quality,nature, etc.) is high. Thus, a polycrystalline SiC intermediate supportmay be used to transfer a high cost and/or high quality monocrystallineSiC film. As an example, the bonding energy may be controlled bychecking the roughness of an additional layer of Si0 ₂ deposited on thethin layer or on the intermediate support. As an alternative to controlthe bonding energy of the intermediate support, it would also bepossible to use a consumable film (for example an oxide) on the surfaceof this intermediate support to recycle it (lift-off technique).

It is easy to choose the thickness of the final buried oxide or theintermediate layer (dielectric, metallic, etc.).

The principle of the invention may be applied to layers made of othermonocrystalline materials made of something other than silicon at leastfor one of the films in the stacked structure. In particular, theprocess may be used to apply a sapphire, SiC, GaN, LiNb0 ₃, Ge, GaAs,InP film onto any support.

This same principle may be applied to target substrate types other thansilicon, for example quartz or any substrate, and advantageously a lowcost substrate (glass, plastic, ceramic, etc.).

This process may be applied to any type of semiconducting film, forexample to III-V, II-VI and IV semiconductors or to a diamond or nitridefilm or any other type of film, for example oxides such as Al₂O₃, ZrO₂,SrTiO₃, LaAlO₃, MgO, Yba_(x)Cu_(y)O_(z), SiO_(x)N_(y), RuO₂ or othermaterials, and particularly piezoelectric, superconducting, insulating,metallic, pyroelectric, monocrystalline and other materials.

This process may be applied to materials with surfaces with polarcharacteristics.

This process may be applied repetitively to materials, for example toobtain complex multi-layer structures.

The manufacturing principle for a monocrystalline film alone, forexample silicon on a final support, may advantageously be used in anapplication in which the final support is a structure with at least oneprocessed or unprocessed layer. This monocrystalline film of transferredsilicon then itself becomes the subject of technological steps in orderto create a component. This principle, if it is repeated, can be usedfor 3D stacking of technological levels of components.

The process according to the invention can compensate for several typesof incompatibilities in a particular production. One or severalintermediate supports may have to be used to achieve this.

Initial substrates, target substrates and intermediate supports may bestacked structures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other advantages and specialfeatures will become clear after reading the following description,given as a non-limitative example, accompanied by the attached drawingsamong which:

FIG. 1 shows a cross-sectional view of a substrate called the initialsubstrate to which this invention is applied,

FIG. 2 shows a cross-sectional view of the initial substrate covered bya material layer and to which an ionic implantation step is appliedduring implementation of the process according to the invention,

FIG. 3 illustrates the step in which the initial substrate is put intobonding contact through the material the material layer with anintermediate support according to this invention,

FIG. 4 illustrates the step in which at least part of the initialsubstrate is eliminated according to this invention,

FIG. 5 illustrates the step of creating a bonding contact with a face ofa target substrate, through a possible layer according to thisinvention,

FIG. 6 shows the stacked structure obtained by the process according tothe invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Several examples of the use of this invention will be described. Theseexamples are variations on a process for embodiment of the invention,consequently we will start by briefly describing this process using anexample and with reference to FIGS. 1 to 6.

FIG. 1 shows a cross-sectional view through a 725 μm thick 200 mmdiameter silicon wafer 1 forming the initial substrate. Face 2 of theinitial substrate is oxidized to a thickness of 400 nm by heat treatmentto create an oxide layer 3 (see FIG. 2). A multi-layer can also be usedon substrate 1.

The next step is an ionic implantation of the initial substrate 1 bygaseous compounds 4 passing through its face 2, and therefore alsopassing through the oxide layer 3 as shown in FIG. 2. For example, thegaseous compounds 4 may be hydrogen ions implanted at an energy of 75keV and at a dose of about 6×10¹⁶ atoms/cm². This thus induces aweakened layer 5 in the plane parallel to face 2.

The part of the substrate 1 between the face 2 and the weakened layer 5forms a film 6. The assembly formed by stacking the film 6 and the oxidelayer 3 forms a thin layer 7. The thin layer 7 has a free face 8 calledthe first contact face.

The first contact face 8 is cleaned to make it suitable for molecularbond, for example by a preparation intended to make it sufficientlyhydrophile, and it is put into contact by molecular bonding with a face11 of another silicon wafer 10 called the intermediate support as shownin FIG. 3. In one alternative application, a layer (adhesive, meltablelayer, etc.) that can cause bonding with the intermediate support may beapplied to bond to the said support.

The assembly consisting of the initial substrate 1 and the intermediatesupport 10 is subjected to a separation treatment, for example a heattreatment, in order to separate the film 6 by fracture from theremainder 9 of the initial substrate 1 (see FIG. 4). The film 6 remainsattached to the intermediate support 10 and has a free face 12.

Advantageously, the thin layer 7 is bonded onto the intermediate support10 and is reinforced by a high temperature heat treatment. Depending onthe temperature, the molecular bonding energy between these two partsmay for example be of the order of 1.5 J/m².

The free face 12 of film 6 is then smoothed by a surface treatment, forexample by mechanical-chemical polishing, or annealing possibly under apartial or total hydrogen atmosphere, bombardment by isolated orconcentrated ions or chemical attack in order to remove all or part ofthe film 6, thus freeing a face of the layer 3 if the layer 3 issufficiently selective with respect to the intermediate substrate. Itmay be covered by a film 13 that may be multi-layer, and the free faceof which forms the second contact face 14 (see FIG. 5). The film 13 maybe obtained by deposition, by heat treatment, by chemical treatment,etc.

After the second contact face 14 has been cleaned, it is put intobonding contact with a target substrate 15. The bond may be reinforcedby a high temperature heat treatment.

The intermediate support 10 is then removed, for example by grindingtogether with a chemical attack. The oxide layer 3 may also be removedto obtain the stacked structure shown in FIG. 6 and composed of thetarget substrate 15, the film 13 and the film 6.

According to a first alternative embodiment, the film 13 may be a 20 nmthick oxide layer produced thermally on the surface 12 of the film 6.The target substrate 15 may be made of silicon and oxidized on thesurface (for example over 20 nm) or not. In this case, the bond of film13 on the target substrate 15 may be reinforced by a treatment at 1100°C. or a molecular type bond. As before, bonding is possible using anadhesive or meltable material or any other type of material. This wouldthen give a high quality stacked structure while avoiding the appearanceof bubbles at the bonding interface. The intermediate support 10 isremoved by grinding followed by a chemical attack in atetramethylammonium hydroxide (TMAH) or potash solution, the oxide layer3 then acting as a chemical etching stop layer for the silicon. Thisoxide layer 3 is eliminated using a hydrochloric acid based solution.The final thickness of film 6 is adapted by thinning, for example usingsacrificial oxidation. A final thickness of 50 nm may be obtained with avery good uniformity.

According to a second alternative embodiment, the oxide layer 3 formedon the initial substrate 1 is 400 nm thick. Hydrogen atoms are implantedunder the same conditions as before. After separation of the siliconfilm 6 from the rest 9 of the initial substrate 1, the film 6 isthinned, by example by sacrificial oxidation to 30 nm, and is covered bya 50 nm thick oxide 13. The second contact face 14 is bonded to thetarget substrate 15. Treatments requiring a high temperature can then beapplied without creating a risk of bubbles appearing at the bondinginterface.

A third alternative embodiment can give a low energy bonding interfaceon the target substrate. In order to achieve this, the surface 12 offilm 6 revealed by separation of the film 6 from the rest 9 of theinitial substrate (see FIG. 4) is smoothed, for example bymechanical-chemical polishing. This thermally forms an oxide film 13(see FIG. 5) with a thickness of 1000 nm. The free surface of film 13 isthen roughened with an average RMS value equal to 0.6 nm, for example byetching using a 10% hydrofluoric acid solution for 12 minutes. Aftercleaning, the second contact face 14 is bonded to the target substrate15. This target substrate is a silicon substrate that may have beenoxidized on the surface, for example to a depth of 1000 nm. Its surfacemay have been roughened to an average RMS value of 0.6 nm using the samechemical treatment as was used for the film 13. The bonding energy isadaptable as a function of the induced roughness and a heat treatment,if any. At this stage, it is possible for the bonding energy to be lowerthan the energy necessary for satisfactory execution of the directprocess, called the threshold bonding energy.

The silicon intermediate support 10 is then removed, for example bygrinding together with chemical attack in a solution of TMAH or potash,the oxide layer 3 acting as an etching stop layer. This oxide layer 3 iseliminated by etching using a hydrofluoric acid solution and the finalthickness of film 6 is adapted by thinning, for example by sacrificialoxidation.

This alternative embodiment is capable of producing a final film 6 withan approximate thickness of 200 nm and with very good uniformity. Thebonding energy with the target substrate 15 is low, so that the film canbe recovered at the end of the process by separation from the targetsubstrate 15. Advantageously, before the film is recovered, it ispossible to make all or part of a component, for example formicroelectronic, optoelectronic, photovoltaic applications or forsensors, etc. Advantageously, according to another application, thistype of stacked structure with low bonding energy can be used to makefilm depositions or transfers. Another application of this alternativeis related to high cost substrates.

According to another alternative, an attempt will be made to reuse theinitial and intermediate substrates. According to this alternative, theinitial wafer may be made of high quality and/or high costmonocrystalline silicon, for example a 300 mm diameter wafer. The targetsupport may be made of lower quality monocrystalline silicon, or lowcost polycrystalline silicon. The obstacles overcome using this processinclude bonding defects or the appearance of observable bubbles in thedirect process. In step d) in the first example, the free face of thethin layer may be bonded directly to the target substrate, for examplein the case in which the application requires conducting bonding. In asecond example, in step d), the free face of the thin layer or thetarget substrate may be covered by an oxide film, for example to enablebonding on the low cost target substrate. In this final example, theoxide makes it possible to smooth the surface to be bonded and thetarget substrate will be made of polycrystalline silicon.

In this alternative to the process, the intermediate silicon substratemay be low cost, for example polycrystalline. An additional layer, forexample made of SiO₂ for surface smoothing, will advantageously bedeposited on the intermediate substrate.

Also in this alternative, the intermediate substrate may be made of highquality silicon. It will then be advantageous to recover it, for exampleusing a lift-off technique or by implantation of gaseous compoundsfollowed by separation, or for example by a technique after step b)based on a fairly low bonding energy to enable separation, for exampleby mechanical or pneumatic methods, etc.

In all cases of this alternative, the initial high quality and/or highcost substrate, for example 300 mm diameter silicon, will advantageouslybe recovered, for example using gaseous compound implantation in step a)and separation from the rest of the initial substrate in step c).

A fourth alternative embodiment can provide a low energy bondinginterface on the target substrate so that the film can be released andthe target substrate can be recovered. This alternative embodiment,which has many points in common with the previous alternative, makes itpossible to apply a number of treatments to the film fixed to the targetsubstrate, for example technological steps for making electroniccomponents including high temperature heat treatments. These heattreatments would make it almost impossible to separate a film from asubstrate produced using the direct process.

We have already seen that it is possible to achieve a low energy bond ofthe film on the target substrate, for example an energy of 0.5 J/m².This type of bonding energy can be achieved despite heat treatment at atemperature exceeding 900° C., by maintaining control over the roughnessof surfaces before making contact to ensure that RMS values are greaterthan 0.6 nm. These energies are then compatible so that the “processed”film can be released by separation of the target substrate afterapplication of the technological steps to make components. It is thenpossible to recover the target substrate, which may be an advantageconsidering its cost.

Consider the 300 mm diameter silicon substrate as an example of a highcost target substrate. Only the film part of the substrate can be reusedfor the components and it may be important to recover the initialsubstrate and/or the intermediate support and/or the target substrate.

A fifth alternative embodiment is applicable in the case in which thefree surface of film 6 delimited in the initial substrate is such thatthe direct bond on a target substrate is very weak, for example due toroughnesses over at least one of the two surfaces to be put intocontact. The bonding energy on the target substrate is then insufficientto enable separation of the film using the direct process.

This problem is solved by preparing the first contact face 8 to make itsuitable for bonding, for example by the deposition of a sacrificialfilm, after the ionic implantation step, and by an additional smoothingor planarization step. According to the invention, an intermediatesupport is used, for example a silicon substrate possibly covered by anoxide layer (for example 100 nm thick) and with a slightly roughenedcontact surface (for example with an average RMS value equal to 0.2 nm).The fact that bonding is applied on a surface with a low roughness valueresults in a sufficiently high bonding energy so that the step in whichthe film 6 is separated from the initial substrate is possible later.

Once the separation has been achieved, a high temperature heat treatmentmay be applied, for example to reinforce the bond or as a function ofoperations necessary for the target applications (diffusion of implantedelement, oxide film deposited later, etc.). A surface treatment, forexample mechanical-chemical polishing, smoothes all or part of therevealed surface of the film separated from its initial substrate.

After cleaning, the free surface of the film 6 or the additional film13, called the second contact face 14 is bonded onto the silicon targetsubstrate that for example is covered by an approximately 1000 nm thickoxide layer, the surface of which is made rough by chemical treatment.Bonding may be varied by a heat treatment, but it remains low. At thisstage, the bonding energy between the film 6 or the film 13 and thetarget substrate 15 (or possibly its surface layer) is lower than theenergy necessary for smooth execution of the step in which the film isseparated from the rest of the initial substrate. The intermediatesupport 10 is then removed, for example by grinding and chemical attackand the oxide layer 3 is eliminated by chemical attack. The finalthickness of the film 6 is modified by thinning, for example bysacrificial oxidation. This thickness may be 50 nm with very gooduniformity.

Advantageously, this fifth alternative will be used when the initialsurface roughness of faces 2 or 8 corresponds to a topology etched onthe initial substrate or on the initial thin layer or on the targetsubstrate. An oxide film 13 (see FIG. 5) with a thickness of 1000 nm maybe made, for example thermally on the surface of film 6. The surfacetopology, that does not initially enable the direct process, may bereproduced at this step, for example by a chemical etching treatment.One application example is the case of ducts placed at the bondinginterface with the target substrate and capable of inducing cooling ofthe structure by circulation of a fluid. Another example in anotherfield is the production of a texture at the bonding interface of thetarget substrate for photovoltaic applications. Furthermore, thistopology may be made exclusively or partially in the target substrate,and may or may not be covered by additional layers.

A sixth alternative embodiment differs from the previous alternative inthat the intermediate support can be recovered. This solution isattractive since the intermediate support is an element with a certainquality to enable separation of films and its cost can then be high. Forexample, the process according to the invention may advantageously keepthe bonding energy of the initial substrate onto the intermediatesupport equal to a value just higher than the threshold energy necessaryto separate the film from the initial substrate. The bonding energy ofthe initial substrate and the intermediate support may be controlled bychecking the surface roughnesses and heat treatments used, if any, inaddition to the separation treatment. Considering that the intermediatesupport can be recycled, it is possible to use expensive substrates assupports (compatible with the application), or even substrates that arespecially prepared to facilitate separation at the first bondinginterface. The intermediate support may be recovered after bonding tothe target substrate using a “lift-off” technique, or by mechanicaland/or pneumatic separation, or by using a process in combination with agaseous implantation, these techniques possibly being combined with eachother.

A seventh alternative embodiment is applicable particularly on the casein which the free surface of the film separated from the initialsubstrate is difficult to polish or if its smoothing quality is notsufficient after a direct process.

The surface roughness of films obtained by the direct process frequentlyhas to be reduced depending on the planned application, after theseparation step. Conventionally, a mechanical-chemical polishing ispossible. However for many materials for example “hard” materials(sapphire, SiC, diamond, etc.), this polishing is either not reallyappropriate (not effective for polishing a film of hard material sinceit was developed for the same material in massive form, or because thequality is insufficient, or if there is a thickness uniformity defect),or it takes too long (which increases the manufacturing cost). Theinvention solves this problem.

Consider the example of an initial substrate 1 formed by a wafer ofmonocrystalline sapphire with orientation [1-102] and polished on thesurface with an epitaxial quality. The wafer may be covered again with alayer of silicon oxide. The initial substrate 1 is implanted by gaseouscompounds, for example hydrogen. If there is no oxide layer, theimplantation energy may be 60 keV for a dose of 2×10¹⁷ atoms/cm². In thepresence of an oxide layer, the implantation energy is increased to takeaccount of the thickness of this oxide layer. After preparation of theimplanted face (first contact face), the initial substrate is put intocontact by molecular bonding with the intermediate support. The sapphirefilm is separated in or close to the weakened layer.

After this separation, it is desired to obtain a sapphire film with lowmicro-roughness. Mechanical-chemical polishing is very long to apply forthis type of material, and the quality and uniformity of polishing athin film is difficult to control. The direct process cannot be appliedsince the surface quality is not of the epitaxial type or the impliedextra cost is high. The sapphire initial substrate is sold by the wafersupplier with a face that is already of epitaxial quality, consequentlythe invention can be used to obtain a stacked structure comprising afilm for which the free face (or front face) is this initial face withepitaxial quality.

Therefore, the separation step between the film and the rest of theinitial substrate is made after putting the initial substrate intobonding contact on the intermediate support through an additionalinitial film. After separation, the free face of the sapphire film has acertain roughness. A layer of material, for example a layer of SiO₂ isdeposited on the free face and mechanical-chemical polishing enablesplanarization of its surface. After preparation of this surface and thecorresponding surface of the target substrate (for example made ofsilicon), the second step of creating a bonding contact is achieved.Removal of the intermediate support reveals the initial front face ofthe sapphire film or the additional initial film. This additionalinitial film may advantageously be a silicon oxide film. In this case,it may be removed by chemical attack to release the initial front faceof the sapphire film. If this front face is covered by an oxide layer,this layer may be removed by chemical attack.

An eighth alternative embodiment is applicable to the case in which thefilm is provided with faces with different characteristics. This is thecase mentioned in the state of prior art for continued growth in epitaxyon the film 6, for example made of SiC (material with an Si type faceand C type face) or a GaN film.

For example, an initial SiC substrate is covered by an oxide layer about400 nm thick. The initial substrate is implanted through the oxide layerby hydrogen atoms with an energy of 120 keV and at a dose of 8×10¹⁶atoms/cm². The implanted surface is then made hydrophile and is put intocontact for molecular bonding with one face of an intermediate support,for example covered by a 1 μm thick oxide layer. A separation treatmentseparates the film from the rest of the initial substrate. The SiC filmthen bonds to the intermediate support through the oxide layer. Asurface treatment (for example mechanical-chemical polishing ordeposition of a film enabling planarization) makes the new free surfaceof the SIC film suitable for subsequent bonding. This free surface ismade hydrophile and is put into contact for molecular bonding with acorresponding face of the target substrate. After a high temperatureheat treatment intended to reinforce bonding, the value of molecularbonding energy can be equal to or greater than 1 J/m².

The intermediate support 10 is then removed, for example by grindingfollowed by chemical attack, the oxide layer 3 acting as an etching stoplayer. Finally, the oxide layer 3 is removed by attack using a solutionbased on hydrofluoric acid. The final thickness of the film is adapted,for example using a thinning heat treatment.

In this alternative embodiment, a final thickness of the film 6 equal to100 nm is obtained with very good uniformity over a large part of thestructure. The released surface of the film corresponds to the surfacesuitable for continued growth in epitaxy.

A ninth alternative embodiment is applicable to the case in which it isdesired to recover the intermediate support and in which the film (orone of the films) has faces with different characteristics. Thisalternative is a special case of the previous alternative.

After the step that consists of separating the film 6 from the rest 9 ofthe initial substrate (see FIG. 4), an ionic implantation step iscarried out through the surface 12 to induce a weakened area in theintermediate substrate or in one of the additional layers deposited onthe intermediate substrate or the thin layer 7 depending on theirnature. It may be a hydrogen implantation in the intermediate substratewith an energy of 140 keV and at a dose of 8×10¹⁶ atoms/cm² for theexample of the materials mentioned in the eighth alternative and inwhich the intermediate substrate is made of SiC.

The target substrate is put into bonding contact with the second contactface. The intermediate support is then separated from the stackedstructure and may be recycled. Recovery of the intermediate SiC supportis economically very attractive in the case of a process for productionof an SiC film on a silicon target substrate. The oxide layer 3 isremoved by etching using a hydrofluoric acid solution. The final filmthickness 6 is adapted by thinning, for example by sacrificialoxidation.

A tenth alternative embodiment applies to the case in which the film andthe target substrate have at least one characteristic that makes thedirect process incompatible. For example, it may be the case in whichthe difference between the coefficients of thermal expansion of thematerials from which the film and the target substrate are formed is toogreat. For example, silicon and quartz, silicon and sapphire, siliconand gallium arsenide, Si and InP, Si and LiNbO₃. A heat treatment usedbefore or during the separation step by the direct process causes eitherseparation at the contact interface or failure of one of the twoelements put into bonding contact.

For example, the starting point can be an initial substrate 1 composedof a silicon wafer covered by a 400 nm thick oxide layer 3. A weakenedlayer 5 is created by hydrogen implantation at an energy of 75 keV andwith a dose of 6×10¹⁶ atoms/cm². The contact face 8 is bonded to anintermediate support 10 with a compatible coefficient of thermalexpansion. This intermediate support may be another silicon wafercovered by a 200 nm thick oxide layer. Heat treatments can then beapplied. These heat treatments enable an increase in the bonding energy,which will cause a high quality separation between the film and the restof the initial substrate. Once the separation has been made, the stackshown in FIG. 4 can be obtained. A surface treatment minimizes thesurface micro-roughness of film 6. After surface preparation, ifnecessary, the stack is bonded onto a target substrate, for which thecoefficient of expansion may be very different from the coefficient ofexpansion of the initial substrate, for example a quartz or sapphireplatelet and the intermediate support is removed, for example bygrinding, chemical etching, lift-off, etc.

Another example of this tenth alternative consists of causing bonding instep b) in which the energy corresponds at least to the threshold energybelow which step c) cannot occur. Before step d), an ionic implantation,for example of hydrogen, is made in the intermediate substrate or in oneof the additional layers in this intermediate substrate through face 12.This implantation will induce a weakened layer in this substrate, inwhich the separation will take place during step e). The intermediatesubstrate can then be recovered and reused.

Similar examples may be obtained with initial substrates themselvescomposed of a stacked structure, for example a silicon wafer covered bya nitride layer although the target substrate may be a silicon wafercovered by a thick layer of thermal oxide. The coefficient of thermalexpansion of the nitride film may be greater than 4×10⁻⁶/k, while thecoefficient of thermal expansion of the oxide film is less than 10⁻⁶/k.A high temperature heat treatment used during or after the directprocess, for example to thin the silicon film by sacrificial oxidation,is not compatible for some bonding energy conditions between the nitrideand silica films. In this case, the process according to the inventionsolves the problem. After step c), the thin layer is treated at hightemperature to thin the silicon by sacrificial oxidation. After step d),an additional nitride layer (Si₃N₄) is made on the free face of the thinlayer and the target substrate is covered by an oxide film (SiO₂). Thetwo layers can also be deposited on the free face of the thin layer oron the target substrate. The final stacked structure corresponds to thethinned layer of silicon supported by the two films with very differentcoefficients of expansion.

According to an eleventh alternative, the characteristic that makes thedirect process incompatible may be a phase change occurring in a film.For example, a palladium film put into contact with a silicon substrateenables bonding by forming a silicide due to a heat treatment at atemperature above 200° C. However, at 900° C., this silicide degrades,for example making it impossible to perform a sacrificial oxidation stepat 900° C. in the direct process. The invention solves this problem.

After making a thin layer 7 comprising a silicon film 6 on anintermediate support 10, the silicon film is thinned at 900° C. bysacrificial oxidation, and the palladium film is then deposited afterthe step to smooth the free face 12, and forms all or part of the filmreference 13. The heat treatment to form a bond with the targetsubstrate is then made at a temperature below 870° C., and the bond willbe good quality and the silicon film will be the right thickness.

1-27. (canceled)
 28. A process for manufacturing a stacked structure comprising at least one thin layer bonded to a target substrate, comprising the steps of: a) forming a thin layer by introduction of gaseous species into an initial substrate, to form a weakened layer separating a film from the rest of the initial substrate, the thin layer having a free face called the first contact face, b) bonding the first contact face of the thin layer with a face of an intermediate substrate by molecular adhesion, c) fracturing the initial substrate at the weakened layer so as to expose a free face of the thin layer called the second contact face and opposite the first contact face, wherein said fracturing comprises subjecting the assembly consisting of the initial substrate and the intermediate substrate to a heat treatment, d) bonding a face of a target substrate with at least part of the second contact face, and e) removing of at least part of the intermediate substrate in order to obtain the said stacked structure, wherein said heat treatment causes bonding defects when steps b) and c) are performed using the target substrate rather than the intermediate substrate, said bonding defects resulting from the difference between the coefficients of thermal expansion of the material of the first contact face of the thin layer and of the material of the face of the target substrate.
 29. The process of claim 28, wherein the initial substrate comprises Si, and the material of the face of the target substrate is a material selected from the group consisting of quartz, sapphire, gallium arsenide, InP and LiNbO3.
 30. The process of claim 28, wherein the initial substrate comprises monocrystalline silicon and the target substrate comprises polycrystalline silicon or monocrystalline silicon of lower quality than the silicon in the initial substrate.
 31. The process of claim 28, comprising a further heat treatment applied after step c).
 32. The process of claim 31, wherein the further heat treatment is a high temperature heat treatment to thin the thin layer by sacrificial oxidation.
 33. The process of claim 28, wherein said step e) is performed by mechanical and/or chemical attack.
 34. The process of claim 28, wherein said step e) comprises separating at a second weakened layer formed by introduction of gaseous species through the free face of the thin layer after step c) and before step d).
 35. The process of claim 28, wherein steps a) and c) are such that a roughness of the first contact face of the thin layer and/or the intermediate support is less than a roughness of its second contact face and/or the target substrate, the structure thus obtained in step d) allows later removal of all or some of the intermediate support by bonding at least part of the second contact face with the face of the target substrate and by removal of the intermediate support.
 36. The process of claim 28, wherein the thin layer and/or the intermediate support and/or the target substrate comprise(s) at least one additional layer with one or more contact faces.
 37. The process of claim 36, wherein the additional layer before step d) is provided with all or part of at least one component.
 38. The process of claim 36, wherein the additional layer is composed of an oxide or polycrystalline silicon or amorphous silicon.
 39. The process of claim 28, wherein the bonding contact of the first contact face and/or the second contact face of the thin layer is a result of the use of a treatment enabling bonding contact chosen alone or in combination among the treatments including mechanical-chemical and/or ionic polishing, insertion of an intermediate layer between a contact face of the thin layer and the corresponding intermediate support or the target substrate, heat treatment and chemical treatment.
 40. The process of claim 28, wherein the bonding energy of the bonding contact of the second contact face of the thin layer with the target substrate achieved at step d) enables removal of the target substrate after step e).
 41. The process of claim 28, wherein step a) is done starting from a substrate (1) covered by at least one layer of material (3).
 42. The process of claim 41, further comprising after step e), a step of eliminating the layer of material (3) covering the substrate (1) in step a).
 43. The process of claim 28, wherein the stacked structure obtained at the end of step e) is thinned on the side of the first contact face.
 44. A process for manufacturing a stacked structure comprising at least one thin layer bonded to a target substrate, which comprises the steps of: a) forming a thin layer by introduction of gaseous species into an initial substrate, to form a weakened layer separating a film from the rest of the initial substrate, the thin layer having a free face called the first contact face, b) bonding the first contact face of the thin layer with a face of an intermediate substrate by molecular adhesion, c) fracturing the initial substrate at the weakened layer so as to expose a free face of the thin layer called the second contact face and opposite the first contact face, wherein said fracturing comprises subjecting the assembly consisting of the initial substrate and the intermediate substrate to a heat treatment, d) bonding a face of a target substrate with at least part of the second contact face, and e) removing of at least part of the intermediate substrate in order to obtain the said stacked structure, wherein the thin layer formed in step a) has a thickness which is sufficient for bonding defects to be avoided while fracturing in step c), and further comprising thinning the thin layer after step c) and before step d).
 45. The process of claim 44, wherein after step c) and before step d) the thin layer is thinned down to a thickness which, if the thin layer formed at step a) would exhibit said thickness, would cause bonding defects as a result of fracturing at step c).
 46. The process of claim 45, wherein after thinning the thickness of the thin layer is less than a few tens of nanometers.
 47. The process of claim 44, further comprising a high temperature heat treatment so as to thin the thin layer after step c) and before step d).
 48. The process of claim 47, wherein the high temperature heat treatment is a sacrificial oxidation of the thin layer.
 49. The process of claim 44, wherein step e) is performed by mechanical and/or chemical attack.
 50. The process of claim 44, which comprises forming a second weakened layer by introduction of gaseous species through the free face of the thin layer after step c) and before step d), and wherein removing of at least part of the intermediate substrate comprises separating at said second weakened layer.
 51. The process of claim 44, wherein steps a) and c) are such that a roughness of the first contact face of the thin layer and/or the intermediate support is less than a roughness of its second contact face and/or the target substrate, the structure thus obtained in step d) allows later removal of all or some of the intermediate support by bonding at least part of the second contact face with the face of the target substrate and by removal of the intermediate support.
 52. The process of claim 44, wherein the initial substrate comprises monocrystalline silicon and the target substrate comprises polycrystalline silicon or monocrystalline silicon of lower quality than the silicon in the initial substrate.
 53. The process of claim 52, wherein in step d) the second contact face or the target substrate is covered by an oxide film.
 54. The process of claim 52, wherein the intermediate support is made of monocrystalline silicon, or the intermediate substrate is made of polycrystalline and the process further comprises a step of depositing an oxide layer on the intermediate substrate before step d).
 55. The process of claim 44, wherein the thin layer and/or the intermediate support and/or the target substrate comprise(s) at least one additional layer with one or more contact faces.
 56. The process of claim 55, wherein the additional layer before step d) is provided with all or part of at least one component.
 57. The process of claim 55, wherein the additional layer is composed of an oxide or polycrystalline silicon or amorphous silicon.
 58. The process of claim 44, wherein the bonding contact of the first contact face and/or the second contact face of the thin layer is a result of the use of a treatment enabling bonding contact chosen alone or in combination among the treatments including mechanical-chemical and/or ionic polishing, insertion of an intermediate layer between a contact face of the thin layer and the corresponding intermediate support or the target substrate, heat treatment and chemical treatment.
 59. The process of claim 44, wherein the bonding energy of the bonding contact of the second contact face of the thin layer with the target substrate achieved at step d) enables removal of the target substrate after step e).
 60. The process of claim 59, comprising after step c) and before step d) thermally forming an oxide film on the second contact face of the thin layer and roughening the free surface of said oxide film.
 61. The process of claim 60, wherein prior to step d) the surface target substrate is roughened.
 62. The process of claim 44, wherein step a) is performed starting from a substrate covered by at least one layer of material.
 63. The process of claim 62, further comprising after step e), a step of eliminating the layer of material covering the substrate in step a).
 64. The process of claim 44, wherein the stacked structure obtained at the end of step e) is thinned on the side of the first contact face. 